TextEditor: Fix snippet editor crash introduced in d85a4f615

The SnippetEditorWidget setup is unusual as it doesn't have
an associated SnippetEditorFactory/SnippetEditor creation chain
and requires manual intervention. This was lost in the
conversion to the new construction scheme.

Change-Id: I0919295603432e525c2abd0da762acc8207f996c
Reviewed-by: Christian Stenger <christian.stenger@digia.com>